1. Field of the Invention
This invention relates to a resonator for suppressing acoustic energy in a combustion turbine power plant and, more specifically, to a modular resonator which may be placed at the location within the combustion turbine power plant having the highest acoustic pressure amplitude and, to a method of placing modular resonators throughout a combustion turbine at the location of the highest acoustic pressure.
2. Background Information
To reduce the amount of NOx, emissions from a combustion turbine power plant, it is known to provide a lean-premix fuel to the power plant""s combustor. A fuel-lean premix includes a fuel premixed with a large excess of air. While the fuel-lean premix reduces the amount of NOx, emissions, high frequency combustion instabilities, commonly referred to as xe2x80x9chigh frequency dynamicsxe2x80x9d or xe2x80x9cscreech oscillations,xe2x80x9d result from burning rate fluctuations inside the combustors which consume the fuel-lean premix. These instabilities may create damaging pressure waves caused by burning rate fluctuations. It is desirable to provide a means of acoustic dampening to suppress the acoustic energy.
One commonly used type of dampening device is a Helmholtz resonator. A Helmholtz resonator provides a closed cavity having one sidewall with openings therethrough. The fluid inertia of the gases within the pattern of holes is reacted by the volumetric stiffness of the closed cavity, producing a resonance in the velocity of flow through the holes. This flow oscillation has a well-defined natural frequency and provides an effective mechanism for absorbing acoustic energy.
Prior art Helmholtz resonators have been in the form of monolithic liners extending over large areas of the combustion chamber walls. Such monolithic liners can be subject to high thermal stress due to the large temperature differences that may occur between the combustion chamber liner and outer walls of the combustion chamber. Monolithic liners may also be difficult to install as the components may interfere with other components of the combustion turbine power plant. Because of these conflicts, monolithic liners are typically restricted to use near the head of a combustor. Additionally, the monolithic liners are supported by circumferentially oriented ribs, honeycomb cells, or other means which provide a compartnentation of the area behind the liner. These structures result in complex, sealed compartment vessel configurations which can be costly to fabricate. Other types of resonators, such as flow-through resonators, have been placed on support plates upstream of the combustor assembly, see e.g. U.S. Pat. No. 5,373,695. Such resonators, however, are structured to be placed in the available space on a cover plate, not at the location of the highest acoustical pressure.
There is a need, therefore, for a modular resonator which may be placed at the location(s) within the combustion turbine flow path having the highest acoustical pressure amplitude.
There is a further need for a modular resonator with a structure that is less susceptible to thermal stress.
There is a further need for a modular resonator that is not adversely affected by thermal stress.
These needs, and others, are met by the invention which provides for a modular resonator having a limited size which may be placed anywhere within the combustion turbine flow path. The modular resonator includes a first member having a plurality of holes therethrough, a second member, and a sidewall holding the second member in spaced relation to the first member. In this configuration, the modular resonator is a closed cavity having a plurality of openings on one wall. Preferably, the first member is located along proximal to the combustor assembly downstream of the combustion zone. More than one modular resonator may be located along the circumference of the transition section.
The modular resonator may also be configured with openings in the second member. Preferably, the openings in the second member are coupled to a purging flow of a gas which is at a greater pressure than the gas in the transition section. The amount of purging gas passing through the second member may be adjusted to change the acoustic resistance of the modular resonator.
In an alternate configuration, the combustion turbine may include a resonator support structure located within the combustion turbine""s flow path. The support structure preferably supports a plurality of tubular members axially aligned with the flow path having a plurality of modular resonators disposed thereon. In this configuration, a portion of the tubular members act as the first member of the modular resonator. The second members may be located either inside or outside the tubular members. Again, the modular resonators may be coupled to a purging gas source.